The present invention relates to an exposure control device for an interchangeable-lens camera with which a variety of photographing lenses can be selectively used according to photographing conditions.
Most recent lens-interchangeable cameras have an automatic exposure control capability. In the case of a TTL (through the lens) photometric interchangeable-lens camera, light entering through the photographing lens is measured at a predetermined position in the camera body to obtain a photometric output ip, which is utilized for exposure control.
For exposure control, in general, the exposure value E.sub.v is obtained from the following equation (1): EQU E.sub.v =log.sub.2 ip/(ips-B.sub.v6)+6+S.sub.v +A.sub.vmin -A.sub.vsmin -L.sub.fv ( 1)
where:
ip is the photometric output provided when a photographing lens is used, PA0 ips-B.sub.v6 is the photometric output with respect to B.sub.v6 of a calibrating reference lens, PA0 6 is the B.sub.v value, PA0 S.sub.v is the film speed value index APEX, PA0 A.sub.vmin is the open-aperture F number APEX of the photographing lens, PA0 A.sub.vsmin is the open-aperture F number APEX of the calibrating reference lens, and PA0 L.sub.fv is the amount of correction for a reduction in the average illuminance on a film surface by relative focal plane illuminance. PA0 To obtain the proper exposure value from equation (1), the open aperture F number A.sub.vmin of the photographing lens and the photometric output ip obtained through the photographing lens should satisfy the following relation: EQU .sub.2 A.sub.vmin .varies.1/ip (2)
This will be described concretely. If, for instance, the photometric output obtained through a photographing lens having an open-aperture F number of 2 is 40 nA, and this photographing lens is replaced by a photographing lens having an open-aperture F number of 2.8, the photometric output obtained through the latter photographing lens should be 20 nA. However, in most cameras, the above-described relation is not established.
FIG. 1 is a characteristic diagram indicating open-aperture F numbers in APEX with differences between the photometric outputs of a variety of photographing lenses and the ideal photometric output provided when relation (2) is established. In FIG. 1, the open-aperture F numbers are plotted on the horizontal axis and the differences on the vertical axis. In the case where the photometric output ip obtained through a photographing lens satisfies relation (2), the difference between the photometric output ip and the ideal photometric output should be zero. However, as is apparent from FIG. 1, the actual photometric output differs from the ideal photometric output.
There are various factors which can contribute to the failure to satisfy relation (2). Both the photographing lens and the camera body have such factors.
In the case of the photographing lens, such factors, for example, include:
(1) A first factor attributed to the cos.sup.4 law. That is, different photographing lenses have different focal lengths, thus providing different photometric outputs. PA1 (2) Different photographing lenses have different exit pupil positions. Therefore, with different photographing lenses, the incident light beam can differ in incident angle on the detector, causing different photometric outputs to be obtained. PA1 (3) Different photographing lenses have different vignetting factors, again causing different photometric outputs. PA1 (4) The photometric output depends on the open-aperture F number. PA1 (5) Transmittances differ among lenses.
The total transmittance includes that of the photographing lens. For a TTL photometric operation, the transmittance of the photographing lens causes no problem because photometry is effected at a predetermined position in the camera body after the light beam has passed through the lens. However, in the case where a photometric system outside the camera is used, the transmittance must be taken into consideration.
In the case of the camera body, a significant factor is that optical components such as the focusing screen or pentaprism positioned in the optical path prior to the photometric point do not always have constant optical characteristics. That is, different camera bodies can have different optical characteristics, thus causing different photometric outputs.
Accordingly, it is necessary to correct the deviation from relation (2) caused by the above-described factors.
Heretofore, the amounts of deviation caused by these various factors were not individually taken into account, and instead a single correction datum for each lens was stored in a lens ROM provided in the photographing lens. Otherwise, protrusions were provided on the photographing lens side to change the resistance of a variable resistor on the lens body side to indicate a correction value.
An interchangeable lens camera must of course be able to use a variety of photographing lenses mounted on its body. Therefore, in such a camera, it is necessary to correct the amount of deviation from relation (2) whenever a different photographing lens is mounted on the camera body. On the other hand, in the case where a single type of photographing lens is employed, there are still deviations from relation (2) attributed to the camera body.
Let us consider the case where photographing operations are carried out by using in combination three different kinds of photographing lenses and three camera bodies picked up from different lots although they are of the same model and basic type. In this case, different combinations result in different types and amounts deviation from relation (2).
Furthermore, let us consider the case in which one photographing lens is to be used with a new camera body employing a different photographing system from the camera body presently in use. If, in this case, the old camera body is of the averaging photometry type and the new camera body is of the spot photometry type, the combination of the one photographing lens and the new camera body will almost inevitably result in a deviation from relation (2); that is, it is impossible to perform photographing operations with a high accuracy when the camera body is changed.
Moreover, when a new camera body is used having a different photometry system, a similar problem results.
The inventor has conducted intensive research on the above-described factors (1) through (4) on the photographing lens side and obtained results as illustrated in FIGS. 2 through 5.
FIG. 2 is a graphical representation showing a characteristic curve indicating the effects of the cos.sup.4 law. This graphical representation was formed as follows: First, light from an object of constant luminance was measured at a predetermined position in a camera body through a plurality of a photographing lenses of different focal lengths so as to obtain exposure values. The relations between the focal lengths and the differences obtained by subtracting the exposure values E.sub.v from a certain reference value D are plotted. As is apparent from FIG. 2, the differences depend on the focal length f.
FIG. 3 is a graphical representation showing a characteristic curve indicating the effects of the exit pupil position. In the case of FIG. 3, the distance of the exit pupil from the film surface was changed to determine the level of exposure at various positions, and the relation between the exit pupil position and the differences obtained by subtracting the exposure level E.sub.v from a certain reference value D are plotted. It is clear from FIG. 3 that the differences depend on the exit pupil position.
FIG. 4A is a graphical representation indicating the effects of vignetting factor. In the case of FIG. 4A, a variety of photographing lenses were combined with a camera body, the exposure levels E.sub.v measured, vignetting factor with an image obtained, and the relations between the vignetting factor with an image height set to 6 mm and the differences obtained by subtracting the exposure values E.sub.v from a predetermined reference value D plotted. In FIG. 4A, reference numerals designate the numbers of the photographing lenses used. It can be understood from this graphical representation that, as the vignetting factor decreases, i.e., as the peripheral quantity of light decreases, the exposure level becomes excessive. FIG. 4B was formed by approximation of the characteristic curve of FIG. 4A.
Thus, with respect to focal length, exit pupil position and vignetting factor, it has been found that, by effecting corrections in such a manner that the variations in difference as shown in FIGS. 2 through 5 are eliminated, the establishment of the above-described relation (2) will be achieved.
An investigation was conducted on the effects of the open-aperture F number as follows: a variety of photographing lenses having different open-aperture F numbers were provided, and, for each photographing lens, corrections were made as described above so that the effects of the exit pupil position, vignetting factor and focal length were eliminated. Thereafter, light from an object of constant luminance was measured through the photographing lenses to obtain exposure levels E.sub.v. FIG. 5 is a graphical representation showing a characteristic curve which indicates the relations between open-aperture F numbers and the differences obtained by subtracting the exposure levels E.sub.v from a certain reference value D. It is apparent from FIG. 5 that the differences depend on the open-aperture F number A.sub.v.
With respect to the camera body, it has been found that selecting the deviation from the above-described relation (2) can be reduced by controlling camera bodies separately in each of manufacturing lots of an optical system extended up to the photometric position.